When a discharged pressurized fluid is supplied simultaneously to a plurality of actuators having different magnitudes of loads, the pressurized fluid is supplied only to the actuator having the minimum load and the pressurized fluid is not supplied to other actuators.
Therefore, as shown in FIG. 1, there has been known a system, in which a plurality of throttle portions, such as a first operating valve 2 and a second operating valve 3 are provided in a discharge passage 1a of a hydraulic pump 1 for simultaneously supplying a pressurized fluid to a plurality of actuators, such as a first actuator 4 and a second actuator 5 by controlling open degrees of the first and second operating valves 2 and 3 depending upon magnitudes of loads.
On the other hand, as shown in FIG. 2, there has been known a system, in which pressure compensation valves 6 are provided on the circuits connecting the first and second operating valves 2 and 3 and the first and second actuators 4 and 5 respectively, and a higher pressure between a load pressure of the first actuator 4 and a load pressure of the second actuator 5 is detected by a shuttle valve 7 to supply the detected pressure to pressure receiving portions 6a of the pressure compensation valves 6.
In the system illustrated in FIG. 2, the pressure compensation valves 6 are set for the highest load pressure. Therefore, the pressurized fluid can be supplied to the first and second actuators 4 and 5 with a flow rate distribution ratio depending upon opening degrees of the first and second operating valves 2 and 3.
In case of the foregoing pressurized fluid supply system shown in FIG. 1, if the load of the first actuator 4 is high and the load of the second actuator 5 is low when simultaneously supplying the pressurized fluid to the first actuator 4 and the second actuator 5, in order to set the pump pressure P0 of the hydraulic pump 1 at a pressure P1 of the first actuator 4 having aa higher load, the opening degree of the second operating valve 3 is reduced to lower the output pressure of the second operating valve 3 to be lower than the pump pressure P0 to be set at the pressure P2 of the second actuator 5.
Therefore, the discharged pressurized fluid of the hydraulic pump 1 causes a significant pressure loss (P0-P2) when passing through the second operating valve 3 and thus a substantial energy loss is caused. For example, a horse power loss of an engine 8 for driving the hydraulic pump 1 will be large.
On the other hand, in a system for controlling a displacement of the hydraulic pump 1 (a flow rate of the discharged fluid per one revolution) depending on the pump pressure P0 to control the horse power, for instance, a system for controlling pump pressure P0.times.displacement to be constant, the pressure P1 of the first actuator 4 having a high load will be the pump pressure P0 to make the displacement small and the flow rate of the discharged fluid of the hydraulic pump small. Thus, a speed of the first actuator 4 will be low.
For example, when P1=100 kg/cm.sup.2 and P2=50 kg/cm.sup.2, the pump pressure P0 becomes 100 kg/cm.sup.2. Then, when a flow rate Q1 to the first actuator 4 is 1 and a flow rate Q2 to the second actuator 5 is 2, a displacement Q1+Q2 of the hydraulic pump 1 becomes 3. Therefore, when pump pressure P0.times.displacement is controlled to be constant, 100 kg/cm.sup.2.times.3 is constant. Thus, the higher the pump pressure P0 becomes, the smaller the displacement becomes.
On the other hand, in the pressurized fluid supply system, as shown in FIG. 2, set fort above, the discharged pressure fluid of the hydraulic pump 1 causes a pressure loss by passing through the operating valve and the pressure compensation valve to cause a significant energy loss to make it impossible to effectively use the discharged pressurized fluid of the hydraulic pump 1.
Therefore, the present invention has an object to provide a pressurized fluid supply system which can solve the problem set forth above.